For effectively performing compression-encoding of digitized moving picture data, widely used are encoding methods that are combinations of correlation elimination/information compression techniques such as interframe predictive encoding, DCT (discrete cosine transform)/inverse discrete cosine transform, quantization, and variable codeword length encoding. As typical examples, there are MPEG-1, 2, 4 modes which are formatted as standards by ISO/IECJTC1/SC29/WG11 (MPEG), and H. 261, H. 263 modes recommended by ITU-T.
(IDCT Mismatch)
As an issue when transmitting moving pictures by utilizing those standardized encoding modes, there is a problem regarding IDCT mismatch errors generated due to inconsistency between the IDCT used on an encoding side and the IDCT used on a decoding side. These are errors generated because requirements for calculation accuracy of the IDCT used in the standard modes are eased so that implementation of an encoding device and a decoding device does not become more difficult than it is supposed to. Specifically, conditions are defined in Non-Patent Document 1 (there is also a regulation in Annex A of Non-Patent Document 2 for MPEG-4), and IDCT is considered to be in conformity to the standards if measures of a maximum value, an average value, and the like of differences with respect to a reference satisfy the criterions even though the outputs of the real-number reference IDCT are not consistent with the numerical values.
A mismatch error generated in one IDCT calculation is merely “1”, and it is so defined that the probability of having an error does not become high. Therefore, an IDCT mismatch generated in one-time picture encoding does not turn out as a big problem. However, when moving picture encoding is conducted by utilizing interframe prediction, mismatch errors generated for each picture are added up and propagated to subsequent pictures. Thus, it is necessary to take a measure so that deterioration of coded pictures due to such error accumulation is not expanded.
A mismatch control is one of the convincing measures. This is a devise for stochastically suppressing generation of output signals that tend to have mismatch errors, through performing signal correction called oddification on input signals in IDCT that is performed when calculating prediction residual signals. The mismatch control has already been employed to the standard encoding modes such as MPEG-2, 4, and the like, and an effect of delaying accumulation of mismatch errors has been achieved.
However, the mismatch control is not a method which can zero the generated errors, so that there is a limit in the effect obtained thereby. Thus, there has been taken a measure which inserts intra-refresh (to cut propagation of errors through conducting intraframe encoding) before the accumulated IDCT mismatch errors become visually conspicuous. Further, with H. 263 mode, there is included a regulation which limits continuation of predictions by conducting intraframe encoding for at least once in 132 pieces of each macro block (Chapter 4.4 of Non-Patent Document 3).
(Problem Regarding Accumulation of Conspicuous IDCT Mismatches Generated in still Scenes)
Noises generated due to the IDCT mismatch errors become conspicuous when still scenes with fewer changes over time are encoded by a small quantization step size. This is because even a small change can be easily detected in a picture where there are only few changes generated as a whole, and noises added to inputted pictures cannot be discarded when the scenes are encoded by a small quantization step size since some kinds of differential signals are encoded constantly and frequency of having mismatch errors generated thereby is increased.
In such conditions, as a phenomenon which particularly causes accumulation of conspicuous IDCT mismatch errors, there is known one condition that is described below. When the encoding side and the decoding side fall into such condition, a fixed IDCT mismatch error is continuously generated in every picture. This fixed IDCT mismatch error is accumulated in a short time, so that an extremely strong noise is included in a decoded picture.
The occurrence state of such problem will be described by referring to FIG. 11A.                An inputted image on the encoding side is perfectly still (fSRC fixed).        The encoding side encodes a quantized coefficient ΔQF that is obtained by performing DCT and quantization on a difference ΔfENC between the inputted image fSRC and a local decoded picture fENC. Since it is a still scene, ΔQF is small. However, a small value (QP=1, for example) is set for the quantization step size, so that ΔQF includes an element other than “0”. However, since it is extremely a small value, an output ΔfENC obtained by performing inverse quantization and IDCT on the inside the encoding side becomes “0” when integerization is performed. As a result, the local decoded picture fENC on the coded side is not updated.        A prediction residual ΔfDEC obtained on the decoding side by performing inverse quantization and IDCT does not match with ΔfENC of the encoding side because of an IDCT mismatch, and obtained is a signal containing an element (±1) other than “0”. Thus, for the picture signals fDEC obtained on the decoding side, signal values change by “1”.        When the still scene continues for several seconds in the above conditions, an error in size “1” of a fixed pattern is added for several tens of times on the decoding side, so that signals that are deviated largely from the inputted picture are generated (see graph of FIG. 1B). The strong noise generated in this manner is highly noticeable, which results in deteriorating the quality of the decoded picture.        
NOTE 2 of 7.4.4.5 chapter of MPEG-4 is a measure for preventing short-time accumulation of such IDCT mismatch errors. This NOTE restricts actions on the encoding side so that the problematic signals are not contained in a bit stream. The NOTE prohibits output of a signal whose prediction residual ΔfENC becomes “0” due to IDCT performed inside the encoding side even through the quantized DCT coefficient ΔQF is other than “0”. When the encoding side conforms to this rule and performs processing for replacing the output ΔQF with “0” when the corresponding signal is obtained, such mismatch error problems do not occur.
However, the regulation of NOTE 2 is a rule recommended to the encoding side, and to follow that rule is not defined as a requisite according to MPEG-4. Thus, unless the encoding side takes any measures, it is impossible to prevent generation of this issue completely on the decoding side. This is the same for the cases that use other encoding modes.
FIG. 12 is an illustration showing a typical structure of a moving picture decoding device. In FIG. 12, only the devices which execute inter-type macro block decoding processing related to the present invention are illustrated, and devices for intra-type macro block processing are omitted. This case includes: a decoding device 101 which decodes an inputted bit stream according to a stream description regulation of MPEG-4; an IDCT device 102 which performs 2D IDCT on a decoded DCT coefficient; a prediction picture generating device 104 which generates an interframe prediction picture according to a decoded result; an adding device 103 which obtains a decoded picture by adding a prediction residual after performing IDCT processing to the prediction picture; and a picture storage memory 105 which provides a reference picture used for encoding following pictures. Through proper operations of those, a decoded picture as defined in MPEG-4 can be outputted to the picture storage memory 105.
Note here that 2D IDCT provided to the IDCT device 102 satisfies an accuracy standard that is defined in Annex A of MPEG-4. However, when an encoding device that does not follow a recommended regulation of NOTE 2 of 7.4.4.5 chapter of MPEG-4 inputs an encoded stream of a still scene and some conditions apply, an IDCT mismatch short-time accumulation phenomenon may occur, which results in obtaining a decoded image containing considerable disturbances.
Non-Patent Document 1: “IEEE Std 1180-1990: IEEE Standard Specifications for the Implementation of 8×8 Inverse Discrete Cosine Transform”, Dec. 6, 1990, pp. 7-12
Non-Patent Document 2: “ISO/IEC 14496-2: Information Technology Coding of audio visual objects, Part 2 Visual”, Jun. 1, 2004, pp. 246, 441-442
Non-Patent Document 3: “ITU-T Recommendation H. 263: Video coding for low bit rate communication”, January 2005, pp. 13